Superluminescent diodes (SLDs) and semiconductor optical amplifiers (SOAs) are opto-electronic devices that provide amplified spontaneous emission of radiation. The devices may be formed by providing a laser cavity structure defining a waveguide that is bounded at opposed longitudinal ends by low reflectivity surfaces. The reflectivity of the surfaces must be sufficiently low to suppress lasing in the cavity.
K. Tateoka et al, “A High Power GaAlAs Superluminescent Diode with an Antireflective Window Structure” (IEEE J. Quantum Electronics Vol 27 No 6 June 1991 pages 1568 to 1573) discloses providing a buried twin ridge substrate (BTRS) structure as part of an SLD structure. An antireflection (AR) coating is provided at a front end of the structure. At a rear end of the structure a window region is provided. The window region is provided by etching through a portion of an active layer of the BTRS structure, forming a current blocking layer, and subsequently regrowing a GaAlAs window region. The GaAlAs window region is arranged to have a mode index to match that of the waveguide.
FIG. 1(a) is a schematic illustration of a known semiconductor laser structure 100 having a laser cavity defined by a self-aligned stripe (SAS) structure 102. The SAS structure 102 is bounded at opposed longitudinal ends by reflective surfaces 103, 104 also referred to as ‘mirrors’.